I. Field of the Invention
The present invention relates to wireless communication devices. More particularly, the present invention relates to a novel and improved method and apparatus for adaptively determining the time necessary to perform reacquisition of a pilot signal in a slotted paging environment.
II. Description of the Related Art
In many wireless communication systems, the mobile receiver is only sporadically active. For example, in a wireless communication system which employs slotted paging, the base station transmits a paging channel which is time-divided into a repeating cycle of slots. Each mobile receiver associated with that base station typically monitors only a single one of the slots in each slot cycle. During the non-assigned slots, the mobile receiver remains in an idle state in which the base station does not send any messages to that mobile receiver, but may send messages to other mobile receivers during their respective assigned slots. In the idle state, the mobile receiver may perform any action not requiring coordination with the base station. Typically, the mobile receiver uses the majority of idle time during non-assigned slots to reduce power consumption by removing power from one or more components, such as those components used for receiving signals from the base station.
This reduced power consumption mode is often referred to in the art as a "sleep" mode. Typically, the mobile receiver will "wake up" from this sleep mode at some time prior to its assigned slot, reacquire the pilot signal of the base station in order to re-synchronize its own internal timing with that of the base station, and then may perform any other actions necessary to prepare it to receive a message at the beginning of its upcoming assigned slot. An example of a wireless communication system which operates in accordance with the above-described principles is given in U.S. Pat. No. 5,392,287, entitled "APPARATUS AND METHOD FOR REDUCING POWER CONSUMPTION IN A MOBILE COMMUNICATIONS RECEIVER," assigned to the assignee of the present invention and incorporated herein by reference.
In a dynamic environment, where the mobile receiver is moving with respect to the base station, the arrival time of the pilot signal at the mobile receiver may change rapidly. This is particularly problematic in a CDMA system, such as that described in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee of the present invention and incorporated herein by reference. In a CDMA system like that of the just-mentioned patent, a shift in the arrival time of the pilot signal at the mobile receiver causes the location of the pilot signal in the PN spreading code to change with respect to the mobile receiver. The longer that the mobile receiver is in the "sleep" mode while moving, the more that the actual pilot signal arrival time may differ from the expected pilot signal arrival time as measured by a freewheeling internal timer inside the mobile receiver. Thus, in a CDMA system like that mentioned above, the actual pilot signal received by the mobile receiver may shift by one or more PN chips while the mobile receiver is "asleep" (i.e. not despreading the pilot signal), resulting in a longer reacquisition procedure because the actual pilot signal arrival time will not match the expected pilot signal arrival time when the mobile receiver "awakens" to reacquire the pilot signal.
In such a dynamic environment, the wakeup time of the mobile receiver before the beginning of the assigned slot must be sufficiently long to allow the mobile receiver to reacquire a pilot signal which may have shifted dramatically in time. In the above-incorporated U.S. Pat. No. 5,392,287, the mobile receiver wakes up a short, but fixed, time before the next occurrence of its assigned slot. This time is generally referred to in the art as "reacquisition time." Empirically, this fixed reacquisition time has been determined to be on the order of about 80 ms. The mobile receiver performs a reacquisition of the pilot signal to realign its internal timing with that of the base station during this reacquisition time prior to the beginning of the assigned slot. An exemplary reacquisition method may be found in copending U.S. patent application Ser. No. 08/696,769, filed Aug. 14, 1996, entitled "SYSTEM AND METHOD FOR RAPIDLY REACQUIRING A PILOT CHANNEL," assigned to the assignee of the present invention and incorporated herein by reference.
In the just mentioned patent, the time it takes to successfully reacquire the pilot channel is roughly proportional to the amount that the actual pilot signal received by the mobile receiver has drifted in time relative to the expected pilot signal arrival time. Thus, the more the actual pilot signal has drifted, the longer it will take to reacquire it. As a result, if a fixed wakeup time before the occurrence of the next assigned slot is used, it must be sufficiently long to allow for the "worst case" drift in the received pilot signal between the time that the mobile receiver entered the sleep mode and the time that it awakens. Otherwise, if the mobile receiver has not yet reacquired the pilot signal before the beginning of the assigned slot, it may miss part or all of a message transmitted by the base station during that slot. If, however, the mobile receiver is relatively stationary, then the actual pilot signal arrival time will be very close to the expected pilot signal arrival time. In this case, successful reacquisition will occur a significant time before the beginning of the assigned slot, and the mobile receiver will merely "wait" for its next assigned slot to begin. During this waiting time, power is applied to all of the receiver components, resulting in significant power consumption, and consequent battery drain.
What is needed is a mobile receiver that "wakes up" from sleep mode just in time to successfully reacquire the pilot signal, without spending extra time waiting for the beginning of the next assigned slot. Furthermore, such a mobile receiver must be able to successfully reacquire the pilot signal in a stationary environment as well as in a rapidly changing dynamic environment. In other words, what is needed is a mobile receiver which adaptively determines the time necessary to perform reacquisition of a pilot signal in a slotted paging environment, without wasting battery power, and also without losing incoming page messages.